The present invention relates to a method for compensation of the reactive power consumption of an industrial load, preferably an electric arc furnace or a plant for rolling of metallic material, supplied from a three-phase electric ac network, wherein a first compensation device for controllable consumption of reactive power, comprising a thyristor-controlled reactor for each of the phases of the ac network, and a second compensation device for generation of reactive power are both connected to the electric power network in a parallel connection with the load, and a device for carrying out the method.
The device comprises means for determining the instantaneous consumption of reactive power by the load, and a control device, which, in dependence thereon, forms a control order for the thyristor-controlled reactors and supplies the control order thereto.
Industrial loads connected to electric ac networks, in particular arc furnaces but also, for example, rolling mills, influence the voltage of the ac network, above all by a greatly varying consumption of reactive power during operation. Especially when the variations in power consumption lie within a frequency interval of typically 0-20 Hz, the corresponding voltage variations, so-called flicker, are observable and disturbing to the human eye in case of electric lighting supplied by the ac network.
It is known, in connection with industrial loads of the above-mentioned kind, to connect in parallel therewith, that is, in a shunt connection to the ac network, static compensators for compensation of the reactive power consumption of the load. One type of such compensators usually comprises a device for generating reactive power as well as a device for controllable consumption of reactive power, known within the art as a thyristor-controlled reactor (TCR). The device for generation of reactive power usually comprises one or more mutually parallel-connected filters, each one essentially comprising an inductive element in series-connection with capacitive elements. The filters are tuned to chosen multiples of the nominal frequency of the ac network, for example to the 3rd, 4th and 5th tones, sometimes even to the 2nd and 7th tones. The device for controllable consumption of reactive power comprises an inductive element, a reactor, in series-connection with a controllable semiconductor valve. The controllable semiconductor valve comprises two controllable semiconductors, usually thyristors, in anti-parallel connection. By phase-angle control of the semiconductors, that is, by controlling their turn-on angle relative to the phase position of the voltage of the ac network, the susceptance of the device and hence its consumption of reactive power can be controlled.
For a general description of thyristor-controlled reactors, reference is made to xc3x85ke Ekstrxc3x6m: High Power Electronics HVDC and SVC, Stockholm June 1990, in particular to pages 1-32 to 1-33 and 10-8 to 10-12.
The compensator generates a reactive power equal to that generated by the device for generation of reactive power, reduced by the consumption in the thyristor-controlled reactor. By determining the instantaneous consumption of reactive power by the load and then controlling the power consumption of the thyristor-controlled reactor to such a value that, together with the consumption of the load, it corresponds to the reactive power generated by the device for generation of reactive power, the reactive power exchange with the ac network becomes zero.
European patent specification EP 0 260 504 describes a circuit for compensation of reactive power comprising a compensator and a load of the above-mentioned kind. In addition thereto, this circuit comprises a self-commutated converter, controlled in pulse-width modulation in dependence on control signals generated in a control member, and connected to the ac network in parallel connection with the load and the thyristor-controlled reactor. The converter supplies to the ac network a reactive current for compensation of the active and reactive power consumed/generated by the load and the thyristor-controlled reactor. In an orthogonal two-phase system, in dependence on sensed three-phase currents and three-phase voltages, the control member calculates instantaneous values of the active and reactive power consumed/generated by the load and the thyristor-controlled reactor together.
In the above-mentioned patent specification, it is stated that the voltage variations in the ac network are substantially determined by variations in the reactive power consumption of the load and that voltage dependence on its active power consumption may be neglected. The control signals to the converter are therefore formed only in dependence on variations in the consumption of reactive power by the load.
The method used in the patent specification EP 0 260 504 for determining the instantaneous active and reactive power of the load in an orthogonal two-phase system is also applicable for control of a thyristor-controlled reactor. However, it has proved to be difficult and in certain cases impossible, with the method for forming a control signal as stated in the above-mentioned patent specification, to meet the increasingly more stringent demands on allowable disturbances imposed by the operators of the ac networks.
European patent specification EP 0 847 612 describes a device for compensation of reactive power in an industrial load of the above-mentioned kind. The device comprises a first compensation device in the form of a thyristor-controlled reactor for controllable consumption of reactive power and a second compensation device for generating reactive power by means of capacitive elements. A control member calculates in an orthogonal two-phase system, via sensed three-phase currents and three-phase voltages, instantaneous values of the consumption of reactive and active power by the load. The control member comprises a signal-processing member with a phase-advancing characteristic within the frequency range of interest to flicker, which signal-processing signal is supplied with a signal corresponding to the mentioned consumption of active power. A control order for the first compensation device is formed in dependence on the consumption of the reactive power by the load as well as on an output signal from the signal-processing member.
It is desirable, among other things because of the expenses, to be able to utilize thyristor-controlled reactors for compensation of reactive power in the applications mentioned in the introductory part of the description, but it has been established that there is a need for further improvements of their ability to rapidly compensate for variations in the power consumption of the load.
The object of the invention is to provide a method of the kind stated in the introductory part of the description, by which the reduction of so-called flicker is improved, and a device for carrying out the method.
According to the invention, this is achieved by sensing, for each one of the phases of the ac network, an instantaneous amplitude value of a voltage which represents the voltage across the thyristor-controlled reactor for the respective phase, by forming a voltage mean value from said amplitude values, by forming, for each of the phases of the ac network, an amplitude deviation as a difference of said voltage mean value and said respective instantaneous amplitude value, and by forming a separate control order for each one of the thyristor-controlled reactors in dependence on the amplitude deviation for the respective thyristor-controlled reactor.
Advantageous further developments of the invention will become clear from the following description and claims.
By means of the invention, the following advantages, among other things, are obtained in relation to the prior art. To obtain a measure of the consumption of reactive power in accordance with the prior art, it is required that sensed measured values of currents and voltages obtained on two separate measurement occasions are supplied to the control equipment. By forming the control order in dependence also on the above-mentioned amplitude deviation in voltage, according to the invention, an influence on the susceptance of the thyristor-controlled reactor may be obtained without waiting for the effect of the voltage change on the reactive power consumption determined in the manner previously known. In addition thereto, it is characteristic of industrial loads of the mentioned kind, and in particular in the case of electric arc furnaces, that variations in power consumption exhibit a considerable phase unbalance. By therefore forming a separate control order for each of the thyristor-controlled reactors belonging to the respective phase, also according to the invention, the effect on the ac network may be further reduced.